[server] improve TZDatetime support by depending on logilab-database 1.15.0 including https://www.logilab.org/ticket/1485893, we can now keep the tzinfo attribute on datetime objects for TZDatetime attributes, so one knows that this is a tz-aware datetime. To easily make it work with backends that have no tz support, we keep converting tz-aware datetime objects into utc naive datetime objects before sending them to the database.

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